1. Field of the Invention
This invention relates to circuits for use in lighting systems employing gaseous-discharge lamps.
2. Description of the Prior Art
Gaseous-discharge lamps have frequently been employed in industrial lighting situations, because of their high efficiency when compared with other light sources, such as incandescent lamps. A gaseous-discharge lamp is a source of radiant energy characterized by the emission of radiation from a stream of ionized current-carrying vapor between the electrodes of the lamp.
The types of gaseous-discharge lamps which are commonly in use today include the mercury vapor, the metal halide, and the high pressure sodium lamps. In operation, several characteristics of each type of lamp are similar. For example, a voltage higher than the normal operating voltage of the lamp is required to ignite the lamp. Once current flows in the lamp, it exhibits a negative resistance characteristic, i.e., the resistance of the lamp decreases with an increase in current through the lamp. Gaseous-discharge lamp lighting systems utilize a ballast to compensate for the negative resistance characteristic of the lamp, and the ballast is connected between the energizing source and the lamp to limit the current through the lamp during normal operation.
One type of ballast which has been used in gaseous-discharge lamp lighting systems is the lead-peaked, regulating ballast. This type of ballast is characterized in that: (1) It is a lead-type ballast; and (2) the ratio of the value of the peak open circuit voltage to the RMS value of the open circuit voltage is higher than for other types of ballasts. Each type of lighting circuit described in this specification may be used in a gaseous-discharge lamp lighting system employing this type of ballast.
While some operating characteristics of all types of gaseous-discharge lamps are similar, other characteristics are quite different. The technique which is required to start the lamp is one example of a characteristic which differs between lamp types. For example, when a gaseous-discharge lamp lighting system employing a mercury vapor lamp is initially energized, no current flows through the lamp and the open circuit voltage of the ballast appears across the lamp. This open circuit voltage is sufficient to enable the lamp to start by means of a starting electrode. Other types of lamps, e.g., the high pressure sodium lamp, do not readily permit the use of a starting electrode. Rather, a high voltage pulse or pulses must be applied to the lamp to start it. It is common for such a lamp to require 50 starting pulses per second, where each pulse has a peak voltage of 2,500 volts sustained for more than one microsecond.
Some time after the gaseous-discharge lamp ignites, it is desirable to terminate the generation of further starting pulses, and starting circuits typically employ some shut-off technique to accomplish this result. The generation of starting pulses may be discontinued immediately upon ignition of the lamp, or may continue until the lamp approaches its normal operating condition and then be discontinued.
Circuits which generate starting pulses for gaseous-discharge lamps have been available, e.g., as disclosed in U.S. Pat. No. 3,681,653 to Snyder. Starting circuits for lighting systems employing lead-peaked, regulating type ballasts have also been available, but these starting circuits typically employ relatively complex techniques to discontinue the generation of starting pulses.
Another characteristic of a gaseous-discharge lamp is that several minutes will pass between the time the lamp ignites and the time the lamp reaches its normal operating condition. The difference in these two times is commonly referred to as the warmup period for the lamp, the the warmup period for a given lamp can vary depending upon a number of factors. During the warmup period, the luminescence of the lamp is not as great as it is when the lamp reaches the normally operating condition. Consequently, auxiliary sources of light, e.g., an incandescent lamp, have been included in some gaseous-discharge lamp lighting systems to provide light during the warmup period. When the gaseous-discharge lamp approaches its normal operating condition, the auxiliary source of light is extinguished. Typically, the auxiliary source of light will not be activated again until the gaseous-discharge lamp fails or has to be reignited following a power interruption.
Auxiliary lighting systems have not generally been employed in gaseous-discharge lamp lighting systems in which the gaseous-discharge lamp requires starting pulses for ignition, because the high voltage level of the starting pulses is detrimental to certain components commonly used in such auxiliary lighting systems.